I've used GA3 to break dormancy on non-cactus seed with 12-24hr soaks, but know that it won't improve viability per se. Any improvement in growth rate above and beyond what's needed to break dormancy usually comes at the expensive of stretching/bolting, I wouldn't bother.

If you do use GA3 on your seeds I recommend you also plant some without treatment for comparison.

I do different amounts for different batches of seeds. I normally will do test batches with 10 seeds in each batch at varying PPM levels and time periods. I usually end up going for a 16-20 hour soak at 500-1000ppm. It all depends on the seed batch. If they are fresh seeds I wont soak at all though. But it actually usually increases germination 2x for me. So I like to do GA3 soaks with most of my seeds (unless I know they were harvested within the last year.)